Distinguish true IgE-mediated allergy from intolerance or a non-allergic rash
Expansion
Take a history covering
- What happened: anaphylaxis, urticaria and angioedema, isolated rash, or nausea and diarrhoea
- Timing: immediate (under 1 hour, IgE mediated) or delayed
- When: many childhood labels reflect a viral rash coinciding with treatment
- Severity: was adrenaline or admission required
- Has any beta-lactam been tolerated since
Not true allergy: nausea, diarrhoea, thrush, headache, or the maculopapular rash of amoxicillin given in glandular fever, which is not predictive of future reactions.
Cross-reactivity with cephalosporins is far lower than the traditionally quoted 10 per cent, at around 1 to 2 per cent, and depends on side chain similarity rather than the beta-lactam ring. Carbapenems have very low cross-reactivity; aztreonam essentially none.
Why delabelling matters: the label leads to vancomycin, quinolones and clindamycin, which are associated with more treatment failure, more C difficile, more resistance, longer stays and higher mortality.
Severe reactions contraindicating all beta-lactams: anaphylaxis, Stevens-Johnson syndrome, toxic epidermal necrolysis, DRESS.